In MIMO wireless communication systems, a Hybrid Automatic Repeat request (HARQ) technique is widely adopted to improve transmission reliability. In a typical implementation that adopts the HARQ technique, a transmitter transmits each data packet with Cyclic Redundancy Check (CRC) bits for error detection. At a receiver, the content of each of transmitted data packet is validated through CRC. If a received data packet fails to pass the CRC validation, the receiver feeds back a negative acknowledgment (NACK) signal to the transmitter to request retransmission. On receiving the request, the transmitter retransmits the precedingly received but unsuccessfully decoded data packet. Subsequently, the receiver combines the retransmitted data packet with the precedingly received data packet to improve decoding performance. On the other hand, if a received data packet passes the CRC validation, the receiver transmits an acknowledgement (ACK) signal back to the transmitter to acknowledge the successful reception and decoding of the data packet.
There are two typical HARQ techniques or data packet retransmission protocols, namely Chase combining protocol and Incremental Redundancy (IR) protocol. According to the Chase combining protocol, a preceding unsuccessfully decoded data packet is simply retransmitted after a transmitter receives a NACK signal. Following the IR protocol, only parity information that differs from what is contained in the preceding transmitted data packet is retransmitted progressively to provide more redundancy for assisting decoding the system information in subsequent retransmissions.
In each IR protocol based retransmission, a transmitter transmits data packets that are not self-decodable. At a receiver, according to the IR protocol, all the data packets (originally transmitted data packets and retransmitted data packets) are combined and decoded at a lower rate. As a result, the IR protocol achieves coding gain for decoding system information by providing more redundancy. In contrast, Chase combining protocol provides energy gain to improve decoding performance by combining the multiple copies of the original data packets.
In practice, the received coded data packets which are not decoded correctly are often stored at the receiver rather than being discarded, and when a retransmitted coded data packet is received, the information from preceding coded data packets and the retransmitted data packet are combined before being fed into a decoder, which increases the possibility of successful decoding.
Transmission capacities of wireless communication systems can be enhanced by the simultaneous transmission of multiple spatial data streams over MIMO communication channels that employ multiple transmit antennas (NT) and multiple receive antennas (NR). The simultaneously transmitted data streams may belong to the one coded data packet (Single Code Word, SCW MIMO) or different coded data packets (Multiple Code Words, MCW MIMO). Here a codeword is equivalent to a coded data packet. In a SCW MIMO system, at a transmitter, a coded data packet is segmented into multiple (spatial) data streams and transmitted via multiple transmission antennas respectively. At a receiver of the SCW MIMO system, all detected data streams are multiplexed into a single data packet that will be passed through a channel decoder and a CRC check module for decoding and CRC validation respectively. Subsequently, an ACK/NACK signal is transmitted to the transmitter of the SCW MIMO system to acknowledge the reception quality of the transmitted data packet. On the other hand, in a MCW MIMO system, each (spatial) data stream belongs to an independently coded data packet. Multiple data streams from multiple coded data packets are transmitted independently from multiple transmitting antennas (NT). At a receiver of the MCW MIMO system, each detected data stream undergoes an independent channel decoding and CRC validation process. A plurality of ACK/NACK signals are fed back to the transmitter to acknowledge the reception quality of multiple coded data packets. In both SCW MIMO and MCW MIMO systems, since the data streams are transmitted by different antennas and travel through different communication linkages (i.e., antennas), the spatial data streams have different degrees of link conditions. As a result, the demodulation quality varies with spatial data streams. The control method of a HARQ process (retransmission protocol, coding rate and spatial processing method, etc) in MIMO systems for retransmissions needs to address possible problems caused by antenna diversity in MIMO systems.
A need therefore exists to provide a retransmission control technique that seeks to address at least one of the problems indicated above.